Characteristic correction system for photoelectric engraving machines



Nov. 19, 1957 M FARBER 2,813,925

CHARACTERISTIC CORRECTION SYSTEM FOR PHOTOELECTRIC ENGRAVING MACHINES \d TONE 34 A a AMPLIFIER [W 52, AMPLIFIER AMPLIFIER MH/TE BLACK L V5 LEVEL 56 rl I l JI/BTRALTION 54 6'6 TONE COMPHVSATOR V 3 mama/v Q g 15!. 2 g E k P/MTOGRAPH/C DENSITY 4 I INVENTOR:

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Nov. 19, 1957 M. FARBER 2,813,925

CHARACTERISTIC CORRECTION SYSTEM FOR PHOTOELECTRIC ENGRAVING MACHINES Filed Dec. 8, 1953 2 Sheets-Sheet 2 7D EXCITED. LAMP Z2 mummy SUBTRACTORW BY Mom/e05 E l/885R,

Arr'Y CHARACTERISTIIC CQRRECTION SYSTEM FOR PHQ'LOEUECTRIC ENGRAVING MACHINES Application December 8, 1953, Serial No. 396,953 3 Claims. (Cl. 1786.6)

This invention pertains to photoelectric engraving machines, and more especially to arrangements by which a desired relationship may be achieved between the signals derived from the photoelectric copy-scanning system and the output of the electrically operated engraving head or stylus motor of such apparatus.

inasmuch as the original of which the engraving is to be made will generally be a photographic print (although it need not be), the relationship between the stated variables may be expressed as the curve or function relating the photographic density variations of the original to the density of a reproduction made from the finished engraving by a specified printing process. Such a curve will be referred to hereinafter as the characteristic of the engraving machine.

By way of explanation, and in order to facilitate a ready understanding, the invention will be described herein specifically in connection with its application to a particular photoelectric engraving machine; however, it is to be understood that it may also be applied to specifically different types of such machines, by modifications that will be apparent to those skilled in the design of such apparatus. The exemplary machine chosen for this purpose is one of known form in which the original and the plate (usually plastic) to be engraved are disposed tightly about a pair of cylinders which rotate in synchronism, together with a pair of carriages arranged to traverse paths parallel to the cylinder axes and carrying, respectively, a photoelectric pickup system which derives a signal from a helical scanning of the original, the signal being amplified and used to drive a stylus motor and stylus which engraves dots in the plate carried by the other cylinder. The dot structure, insofar as concerns its repetition pattern (apart from the variations due to the photoelectric scan signals) is obtained by superimposing upon the photocell output a constant-frequency signal derived from a generator which is synchronous with the cylinder rotation, known as the tone signal due to the fact that it lies within the audio frequency range and has a constant tone.

It has been found that in reproducing or making ongravings from photographic or other originals having a short tone scale (tone is here used in its photographic sense as indicating a range of contrasts), the usual adjustments of the engraving machine produce an engraving in which the reproduction density is lower than it should be in the regions of higher density. That is, the machine adjustment in accordance with readings taken in accordance with a representative blackest area of the original and a representative whitest area, while producing a satisfactory engraving from a normal, full-tonescale original, will have an incorrect tone representation in areas corresponding to higher densities, or in the gray to black regions of the engraving. Another way of stating this is to say that the characteristic of the machine for such originals droops or falls off in the high density region.

States Patent 2,813,925 Patented Nov. 19, 1957 It is the correction of this drooping characteristic that is the principal object of the present invention.

A further object of the invention is to provide a novel method of operating upon the signals passing to the cutter or stylus of a machine such as described, which method will result in correct and proper renditions of the original even though the same be deficient in contrast range.

Another object of the invention is to provide a very simple and relatively inexpensive modification of the known circuitry of a machine of the type described, in order to accomplish the main object set forth above.

The above and other objects and advantages of the invention will best be understood by reference to the following detailed specification of a preferred embodiment thereof, and of the manner of practicing the invention, said specification being taken in connection with the appended drawings, in which:

Fig. l is a schematic view of a machine of the type with which the invention is concerned, and showing the way in which the invention is to be applied thereto,

Fig. 2 is a graphical representation of several characteristic curves for such a machine when reproducing a shortscale original,

Fig. 3 is a partial wiring schematic showing a preferred arrangement for obtaining the desired characteristic for the machine, and

Figs. 4 and 5 are schematic wiring diagrams showing typical forms of adding and subtracting circuits which may be used in connection with the invention.

The invention will now be described in detail in one exemplary embodiment thereof in connection with a known form of photoelectric engraving machine, namely that shown and described in the U. S. patent to Boyajean, No. 2,575,546, issued November 20, 1951, to the assignee of the present invention. Reference is hereby made to said prior patent for additional details of the complete machine, since only the parts essential to an understanding of the present invention are reproduced herein.

Referring to Fig. 1 of the drawings, the engraving ma chine itself is shown diagrammatically as comprising the cylinders 10 and 12, mounted for synchronous rotation upon a common shaft 14 which may be driven as by reduction gearing 16 from a motor 18. One of the cylinders, such as cylinder 10, is arranged to have secured therearound an original, such as a photograph, of which an engraving is to be made. A carriage 20 is mounted to reciprocate along a path parallel to the axis of cylinder it as upon guides (not shown), and carries a suitable light source 22 directed upon a small spot of the original, from which the light is reflected to a photocell 24 also on carriage 20, so that the photocell output is a function of the reflectivity (or reflection density) of the successive regions of the original scanned by the device. A manual clutch 25 is provided to enable cylinder 12 to be driven while cylinder 10 is stationary, and carriage 22 can also be disconnected from belt 30, both for a purpose to be described.

Cylinder 12 also has a carriage associated therewith, the same being designated 26, guided like cylinder it and carrying the magnetoelectric stylus motor and stylus indicated generally by 28. Cylinder 12 carries wrapped around it a plastic or like engraving sheet upon which the stylus cuts, burns, or otherwise forms an engraved image corresponding to the matter shown on the original copy sheet of cylinder 19. As more fully described in the prior patent referred to above, the carriages are connected to reciprocate alongside their respective cylinders in opposite directions as by being connected to opposite runs or passes of an endless flexible and inextensible beltlike connector 30 carried about opposite pulleys 32 and 3 t. One pulley is driven by suitable gearing, denoted by 36, from the same motor or gearing which rotates the cylinder shaft 14. Thus, the photocell system and the stylus system are reciprocated in timed relation to the cylinder rotation, and the illuminated spot and stylus trace out scanning helices upon the layers carried by their respective cylinders.

Since the final engraved plate must carry a representation equivalent to an ordinary screened printing plate for half-tone work, the apparatus includes means for generating a pattern signal governing the engraving motion of the stylus in a regular fashion, such signal being incorporated into the photocell output, as amplified and applied to the stylus motor. Such a generator is shown at 38 as driven synchronously from shaft 14, the details thereof being given in detail in the prior patent. The tone output, suitably amplified by a conventional amplifler 40, is applied to energize the gaseous illuminating lamp 22 over the path 42, and thus to modulate the photocell output voltage which also represents the variations in reflectivity of the original copy on cylinder 10, and the tone signal is also applied through other amplifiers and controls to the stylus motor 28.

The signal applied to the stylus motor is thus a composite signal made up of a component of fixed frequency and constant but adjustable amplitude, from the tone generator and intervening amplifiers and controls, and a component from the photocell and its output circuit which may be thought of as a slowly varying direct current derived from the variations in reflection density of the original copy, superimposed or modulated upon a carrier at tone generator frequency injected by reason of the energization of the exciting lamp 22 from the generator 38. In the patent mentioned above, these two components are referred to as the black level signal and the white level signal, respectively, so called because in making a normal engraved process plate, adjustment of the black level signal will control the minimum penetration of the engraving stylus (that is, in the absence of any illumination from the copy to the photocell), while the white level signal will control the magnitude of the increase in penetration resulting from the scanning of an area of the original which is capable of reflecting any sensible amount of light into the cell.

In adjusting the machine of the patent for engraving from a particular piece of original copy or photograph, the carriage and copy cylinder 10 are adjusted so that the photocell 24 receives light from a blackest part of the copy and clutch 25 is disengaged and carriage 20 disconnected from belt 30, so that only this selected area of the original reflects into the cell. Motor 18 is energized, with carriage 28 so located as to engrave a marginal trial area of the engraving sheet in accordance with the blackest area of the original, and the black level signal (and if necessary the position of the stylus relative to the sheet) is adjusted until engraved trial craters of a specified small size just appear on the sheet as viewed in the stroboscopic viewer of the patent. The White level is then adjusted by setting the scanner spot from lamp 22 on a whitest portion of the original, and repeating the above process to produce trial craters of maximum area, that is, of the greatest size which will still leave a small island or untouched surface of plastic sheet between adjacent penetrations. The pointed stylus produces a penetration whose area is proportional to the penetration depth. With these adjustments, the apparatus makes a proper engraving from the original, if the latter is of normal tonal range, that is, has a normal range of reflectivities extending from substantially pure black to the reflectivity of white paper.

It has been found, however, that original copy material having less than full tone scale, such as photographs in which the darkest portion has a substantial reflectivity, or in which the lightest portion has considerably less than normal reflectivity, or both, the reproduction density achieved by the engraving falls 0E abnormally in the higher density (darker) portions of the subject matter, if the machine is adjusted .as described. For such subject matter, special compensations have had to be made in order to produce a proper engraving, and these involve considerable judgment and skill on the part of the operator. The present invention provides an automatic compensation for this purpose, and enables the unskilled operator to produce good engravings even from shortscale original subject matter.

The invention is based upon the discovery that the characteristic of the engraving apparatus, when provided with a non-linear tone compensator, is strongly altered by the fact of absence of or reduction in the magnitude of the black level signal. Specifically, if the black level signal is reduced or entirely omitted, the slope of the characteristic curve increases in the higher density regions of the original. The invention makes use of this fact to provide a characteristic which will vary in ac cordance with the tone scale of the original if the normal adjusting procedure is used, the variation being such as to compensate for the drooping characteristic which otherwise would govern the reproduction. Mere modification of the black and white level signals to fit lowcontrast copy, will not produce a satisfactory result, as the reproduction contrast range will be likewise low or unnecessarily limited. According to the invention, such correction can be realized by subtracting from the normal photocell output signal, a controlled portion of pure tone signal as derived from the tone signal generator.

The non-linear tone compensator circuit of the machine (60 of Fig. 1) is a device which compensates for the non-linear effects resulting from the individual nonlinear characteristic of the cutting stylus and its drive; that is, non-linearity as between the reproduction density and the stylus travel. Such a compensator is described in the copending application of I. A. Boyajean, Serial No. 248,755, filed September 28, 1951, and assigned to the assignee of the present application. This compensator circuit 60 is distinguished from the present invention in that the latter provides a further compensation for photographic (original) contrast of abnormally low range, although in accomplishing this purpose the invention does make use of compensator 60 to achieve the desired result. This is because the present invention provides a signal subtracted from the compensator 69 input, and a signal added to the compensator 60 output.

The circuitry by which the above improvement is accomplished will not be described. As stated, the amplified output of the tone generator 38 is applied by path 42 to energize the exciter lamp 22. Another portion is applied to a pair of ganged potentiometers 44 and 46 whose lower ends are shown grounded to complete the return circuit to amplifier 40, one side of whose output will also be grounded. The adjustable contacts of these potentiometers therefore provide for deriving from amplifier 4t) separately adjusted portions of the amplified pure tone signal. The signal from the adjustable contact of potentiometer 44 is conveyed over path 48 through an addition circuit 50 (to be described) whose output passes to the stylus motor 28, a power amplifier 52 being provided if required, as shown in the prior patent. However, instead of merely adding the amplified output of photocell to the input of the power amplifier (as taught by the patent), said photocell output has subtracted therefrom a portion of the amplified pure tone signal derived from the sliding contact of potentiometer 46. The channel from the photocell output amplifier to the subtracting circuit 54 is designated 55, and the channel from the output of potentiometer 46 to said subtracting circuit is designated 58. The output of subtracting circuit 54 then passes through the tone compensating circuit 60 and is added to the output of potentiometer 44 by the addition circuit 50.

The white level signal, adjusted in the prior patent by controlling the portion of the amplified photocell output applied to the power amplifier, is here obtained by controlling the supply voltage to the photocell 24, as by a potentiometer 62 connected to the direct current photocell supply source indicated as a battery 64. Alternatively, the white level signal may be adjusted through control of scanning lamp 22 or by modifying the input or output signal of the photocell amplifier.

The eflect of the above arrangement will be better understood by referring to Fig. 2, which illustrates typical machine characteristics in terms of the relationship between the photographic density of the subject (plotted as abscissae) and the reproduction density of the engraving (plotted as ordinates). Curve A represents the relationship mentioned above as resulting from application of the usual adjusting procedure to an original of short scale, on a machine not equipped with the present invention, and it will be noted that in the higher density region, the slope of this curve decreases with increasing density, as indicated by numeral 66. Curve B shows the effect on the characteristic of removing the black level signal entirely, and it will be seen that this causes the characteristic to rise unduly in the same region, as indicated by numeral 68. The circuitry described operates to derive a portion of the pure tone signal, controllable in magnitude in accordance with the setting of the contact of potentiometer 46 and (since the latter is unicontrolled with potentiometer 44) therefore controlled in accordance with the black signal level delivered to channel 48, and to subtract this portion of the tone signal from the elfective output signal of the photocell amplifier, after which circuit 50 adds together these two components (by addition circuit 50) to provide the final composite signal to the power amplifier and stylus motor. The values of the circuit constants are chosen so that, quantitatively, curves A and B of Fig. 2 are added, with suitable reduction in the total sum to provide the desired characteristic C, in which the decreasing slope of curve A is just neutralized by the increasing slope of curve B in the higher density region 68.

Since, in the process of setting up the machine and making the adjustments of the black and white levels as already described, the setting of the controls in accordance with photocell output from darkest and lightest original copy portions controls the adjustments of potentiometers 44 and 62, and since potentiometer 46 is ganged with the former, the characteristic of the machine for normal or full-scale originals Will remain substantially unchanged (approximating curve C of Fig. 2) by the additional circuitry, because the setting of potentiometer 46 is varied with the setting of potentiometer 44.

A preferred circuit arrangement for deriving from the tone source the desired components (applied to the ganged potentiometers of Fig. 1) is shown in Fig. 3, in which vacuum tube 70 represents the final amplifier stage of tone amplifier 40, energizing the exciter lamp as by an output transformer 72. A second transformer 74 has its primary connected in the cathode circuit of tube 70, and

has separate secondary windings 76 and 78 energizing the respective potentiometers 44 and 46. The remainder of the circuitry may be identical with that shown in Fig. 1.

Fig. 3 shows schematically a typical form which an addition circuit such as 50 in Fig. 1 may take, although other equivalent adding circuits are well known in the art. In this figure, numerals 80 and 82 indicate respective transformers whose primary windings are energized by the respective signals to be added. The secondary voltages of these transformers are merely added by series connection to produce the summation output on output lead 84. In Fig. 4, the same arrangement is employed for subtraction, as in the case of circuit 54 of Fig. 1, by merely reversing the terminals of one secondary winding so that the output of transformer 82 is subtracted from the output of transformer 80. The details of these adding and subtracting circuits form no essential part of the present invention.

While the invention has been described, by way of example, in connection with a preferred embodiment, it is to be understood that many variations thereof, and in its implementation, will occur to those skilled in this art. Therefore, the description above is to be taken as merely illustrative, and the invention is intended to include all such modifications as fall within the scope of the appended claims.

What is claimed is:

1. In an engraving machine including means for scanning original copy and deriving signals therefrom, a tone source for modulating the output of said scanning means, means for controlling a plate-engraving tool in accordance with the modulated scan output signals, and control means for adjusting the depth of cut of said tool when a black portion of the copy is being scanned; a correction circuit for use with copy of low contrast range, comprising means for subtracting from said modulated scan output a voltage derived from said tone source and being a function of the setting of said control means, to provide a difference signal, and means for augmenting the energization of said engraving tool in accordance with the magnitude of the resulting difference signal.

2. Apparatus in accordance with claim 1, including a non-linear compensating circuit energized by said subtracting means and repeating a modified difference signal to said augmenting means.

3. Apparatus in accordance with claim 1, in which the means for controlling said tool comprises a potentiometer connected to said tone source and an amplifier fed from said potentiometer to drive said tool, and a second potentiometer connected to and unicontrolled with the first potentiometer to derive said amplitude-proportional voltage.

References Cited in the file of this patent UNITED STATES PATENTS 

